The present invention relates to a circuit for obtaining a wide dimming ratio from a Royer inverter and, more particularly, to a circuit that allows a back light in an electric device to have a wide dimming ratio without having unwanted current spikes that would significantly reduce the life of the back light bulbs.
Royer inverter circuits for converting a DC electrical signal to an AC electrical signal are well known. For example, these circuits are used when illuminating back lights used in conventional displays found in electronic devices.
Liquid crystal displays, such as those used in calculators and avionics instrumentation, are well known. Some LCD displays utilize aback light consisting of cold cathode fluorescent tubes (CCFTs) to form the display. In certain avionics scenarios, especially in dark conditions within the cockpit (such as when it is dark outside the cockpit), LCD displays become overly bright and the pilot wishes to dim the LCD displays to allow his/her eyes to adjust to the outside conditions.
Traditionally, back lights are dimmed in one of two ways. The first way is to pulse width modulate (PWM) the back light. This can deliver wide dimming ratios but larger starting transients can cause reduced bulb life and other problems such as large current transients in the system. When turn on is slowed to reduce the transients the dimming ratios are also reduced resulting in dimming ratios typically less than 30 to 1 where dimming ratios are defined as the time available for illumination divided by the actual illumination time. The second method is to reduce the current in the bulbs. This method produces limited dimming of the back light and typically results in dimming ratios of less than 20 to 1.
Expanding on the first method discussed above, a pulse width modulated (PWM) signal is used to turn on the circuit for driving the back light that establishes a period of time during which the back light may be turned on. This period is set long enough to provide a wide range of dimming. Dimming is accomplished when the back light is on for a time less than the full period available. This new way of illuminating back lights presented new problems. In particular, the PWM signal turns on the DC-to-AC inverter circuit at a fast rate that causes undesired current spikes in the inverter circuit. These current spikes carry over to the output signal of the inverter circuit and significantly shorten the life of the back light bulbs or back light CCFTs. The current spikes also propagate out of the power supply lines and can cause various system problems.
Accordingly, there exists a need for a display to have a wide dimming range without shortening the life of the back light bulbs or causing system power supply noise. The present invention fills these and other needs, and overcomes the short-comings of the prior art.